1. Field of the Invention
This invention relates to a device for preventing a wheel lock condition from occurring during brake application to a vehicle.
2. Description of the Prior Art
Generally, when a wheel braking force becomes excessively greater than a frictional force between the wheel and the surface of a road during brake application to a vehicle, a preferable rate of slip of the wheel on the road surface no longer can be maintained. Then, the slip rate increases and sometimes it increases to such an extent as to bring about a phenomenon called wheel lock. The wheel lock phenomenon in some cases separately occurs at front and rear wheels or sometimes simultaneously occurs at both the front and rear wheels on a frozen road surface. However, a condition in which a rear wheel locking condition independently takes place is generally regarded as most dangerous because it results in a tail swinging movement of the vehicle.
Heretofore, there have been made various contrivances for prevention of such a wheel locking condition. These contrivances of the prior art include, for example, a device called an antiskid device which restores the rotating force of the locked wheel by promptly lowering brake fluid pressure that has been applied thereto and a fluid pressure control valve such as a proportioning valve, etc. in which the brake fluid pressure on the rear wheel which tends to incur the wheel locking condition due to a shift of a load on the vehicle at the time of brake application is arranged to increase to a lower degree than that of the front wheel in accordance with a prescribed brake fluid pressure distribution ratio for the front and rear wheels.
However, these prior art contrivances have many shortcomings. In the case of the former example of them mentioned above, the reduction device for reducing the brake fluid pressure generally requires use of a considerably large operating mechanism of a pneumatic operating type. This presents problems in terms of space and cost and the device is not usable for general vehicles with the exception of some vehicles of limited kinds. The latter example of the prior art contrivances has an advantage in that it can be arranged into a compact size and at a relatively low cost. However, since this control valve is of the control type having a specific brake fluid pressure increasing characteristic without having any feedback function for feeding variations in the speed of revolution of the wheel back to the fluid pressure control system, the control valve has been incapable of adequately coping with the influence of transitively varying factors such as the condition of the load on the vehicle, the coefficient of friction of the road surface etc.
The present invention is directed to the solution of these problems which have been presented by these wheel lock preventing contrivances of the prior art.